A motion detonator is a small device that sets off a pyrotechnics prop when moved

How motion detonators can be useful

The first time I had to use a motion detonator was when I was the pyrotechnician for a modern version of Shakespeare's A Midsummer Night's Dream. In the piece, Puck is supposed to kick a pipe standing upright, which then starts to release lots of heavy smoke. I was going to remote control this, but putting cables was going to be difficult. Instead, I decided to make a motion detonator, so the prop was self contained (one less thing to worry about)

How to make one

A motion detector like this is actually laughably simple to make. What I did was this

   |  | 
   |  |=====,  <-- Nail driven into the tube
   |  |     |
   |  |     |  <-- Thin copper wire with a metal nut soldered to the end
   |  |     |
   |  |     |
   |  | |   |   |
   |  |=|  / \  | 
   |  | |  \./  |  <- small metal cannister
   |  | |       |
   |  |=|       |
   |  | |_______|
   |  |
   |  |
   |  |^
   |  || 
   |  |`Cannister fixed to tube wall

This little device affixed to the side of the pipe is enough to make a simple motion detector. When the actor kicks the tube, the copper wire oscillates, hitting the side of the metal cannister. All of this is wired like this:

|                       |
|,-----,                |
||     |                |
|--,   |                |
|  |   |                |
|| | | |                |
|| O | |                |
||___| |          |¨¨¨¨||
| |    |          |    ||
| _    |          |    ||
|| |   |          |    ||
||A|   |          |  B ||
||_|   |          |    ||
| |    `--------- |    ||
| `-----------------___||
|                       |

A - 9V battery 
B - Smoke cartridge

When the nut-and-wire moves, the electric cirquit is completed, igniting the smoke cartridge which then expels a massive amount of smoke for a few seconds.

A word of advice

Obviously, this is a self contained system. That means that if you decide not to use the prop, you are in trouble - you would have to have very steady hands to "disarm" the prop by lifting the pendulum out of the cannister. Equally, when you arm the device, it is easy to set it off by accident. It is not very nice to get a face full of smoke, although it is not really dangerous. If you were to use this with explosive props, however, we are talking a factor of serious danger.

Usually when I make these kinds of props, I include a safety loop. In other words, inside the cannister, you add a switch and a LED. The LED is coupled straight to the battery, and lights up when the button is on. This way, you can easily determine if the prop is "armed" or not. Also, you can prepare the prop completely, and just turn on the "button" when everything is ready. This also allows stage crew to put the prop in place (it can't be standing around armed all the time - what if somebody trips over it?), make sure the pendulum hangs straight, and just flick the switch to arm the prop.

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Please read the disclaimer. Also, make sure you have read the Pyrotechnics safety tips. SAFETY FIRST

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